This invention relates to ATMs and, more particularly to circumventing of faults in I/O modules of an ATM.
FIG. 1 presents a general block diagram of a conventional local ATM switch 100 with a connected I/O module 10, and conventional remote ATM switch 200 with a connected I/O module 20 (that may be of the same construction as that of module 10). Module 10 contains a line interface unit (LIU) 110 that is connected to fiber 210, and a line interface unit 120 that is connected to fiber 220. Fiber 210 is the xe2x80x9cservicexe2x80x9d line, in the sense that it carries live data between I/O module 10 and I/O module 20. Fiber 220 is the xe2x80x9cprotectionxe2x80x9d line, in the sense that it is ready to assume the active communication function of line 210, should fiber 210 fail. Within module 10, LIU 110 is connected to framer 111, and framer 111 is connected to APS switch unit 130. Similarly, LIU 120 is connected to framer 121, and framer 121 is connected to APS switch unit 130. APS switch 130 is connected to ATM processing unit 140, and the output of ATM processing unit 140 forms the output of I/O module 10. This output is connected to ATM switch fabric 100. Elements 111, 121, 130 and 140 are connected to a control CPU 150. Additionally, CPU 150 includes an ATM bus through which the CPU communicates directly with switch fabric 100 (not shown explicitly).
Under normal operating circumstances, traffic from the service fiber (210) passes through LIU 110 and framer 111, and is applied to APS switch unit 130. The switch is set to pass this traffic to ATM processing unit 140 and thence, to ATM switch fabric 100. In the reverse direction, traffic flows from switch fabric 100 to ATM processing unit 140, and is bridged by APS switch unit 130 to both framers 111 and 121. That traffic is then transmitted out on both fibers 210 and 220. From the above it can be realized that protection fiber 220 carries signals that are identical to the signals carried in service line 210. The only difference is that APS switch 130 in I/O module 10 passes only the signal of framer 111 to switch unit 140 and, similarly, I/O module 20 at the remote destination passes only the signal of framer 123 to switch unit 145.
When a failure occurs, for example, when fiber 210 is severed, CPU 150 gets an interrupt signal via line 151 from a detector in framer 111. In response thereto, the CPU takes recovery action. First, the CPU checks to determine whether the protection line (220) is in good operating order. Upon an affirmative determination, CPU 150 orders APS switch 130 to disconnect the path from line 210 toward ATM processing unit 140, and to connect the path from line 220 to ATM processing unit 140. CPU 150 also creates an APS signal and casts it onto line 220 through framer 121, toward I/O module 20. Framer 113 at I/O module 20 provides the received APS signal to CPU 160, and CPU 160 directs APS switch unit 135 to switch the signal arriving on fiber 220 to ATM processing unit 145.
In may be noted that fibers 210 and 220 may each be a pair of fibers for carrying the two-directional traffic, or they may each be single fibers (with the two channels multiplexed thereon using, for example, wavelength division multiplexing).
While an ATM constructed with I/O modules as shown in FIG. 1, and employed in the manner described above, is able to circumvent problems that originate in the fiber or the LIU, it nevertheless had a significant weakness. Use of the APS switch within the I/O module requires one to connect the service fiber and the protection fiber to the same I/O module. Consequently, a general failure in the I/O module brings down both the service path and the protection path. On first blush, it would appear that placing the APS switch off the I/O module, in a separate circuit board that is interfaced between the I/O module and the ATM switch, would solve the problem because it would allow the service fibers and the protection fibers to be connected to different I/O modules. Alas, current design ATMs do not have the physical room for inserting the circuit board that would serve as the switches for selecting I/O modules. Moreover, such a solution is quite expensive.
An improved arrangement is realized by operating in a novel manner that allows the connection of the service fiber and the protection fiber to different I/O modules and achieving the necessary switching functions without the need of additional circuit boards. More specifically, while the service line and the protection line are connected to different I/O modules, the selection of the service line or the protection line is carried out by cooperation between the CPUs on the I/O modules of the service and the protection lines and the ATM switch fabric. The line that is selected has its framer buffer open, while the line that is in the standby mode has its framer buffer closed. In the other direction, traffic is multi-cast onto both the service and the protection lines by the ATM processing unit. In this manner, the protection fiber always contains information, ready to be switched from standby mode into active mode.